Recently, the performance of semiconductor devices, such as semiconductor integrated circuits, has been improving without showing any sign of leveling off, and the need for further integration and high-density packaging is becoming more and more compelling. For example, one problem of microfabrication for further integration and high-density packaging is a problem related to an insulating film, such as a gate insulating film and a capacitor insulating film, of an MOS transistor, an MOS capacitor, and the like.
In a device (such as a silicon device) formed by using a silicon substrate, and in an MOS transistor and an MOS capacitor in particular, a silicon dioxide film is usually used as an insulating film, such as a gate insulating film and a capacitor insulating film.
As a result of microfabrication of the device, the insulating film becomes extremely thin. For example, if the design rule is not more than 0.07 μm, the gate insulating film is required to be not thicker than 1.5 nm.
However, considering such problems as the increase of leak current, it is estimated that the limit of microfabrication of silicon dioxide film is 1.5 nm to 1.2 nm. In view of this, the use of such highly dielectric materials as Al2O3 and Ta2O5 is considered, but by they are not yet used practically. Even if these highly dielectric materials are put into practical use, the introduction of these new materials will require a great amount of initial investment.
Conventionally, the oxide film (gate oxide film) used as a gate insulating film of an MOS transistor has been formed by heating a silicon substrate at a high temperature of not lower than 800° C. in an oxidizing gas such as dry oxygen or vapor. However, if an extremely thin oxide film of not thicker than 2 nm is formed by high-temperature thermal oxidization, there is a problem that the oxide film cannot be used as a gate insulating film because of its high leak current density. In addition, it is difficult to control the thickness of the oxide film formed by high-temperature thermal oxidization, because the initial growth rate of the oxide film is high. Therefore, it is difficult to form an extremely thin oxide film by high-temperature thermal oxidization. Moreover, the high-temperature thermal oxidization also has a problem that the dopant diffuses when heated at a high temperature, resulting in the destruction of shallow junctions.
Other than high-temperature thermal oxidization, there are such methods as chemical vapor-phase growth (in which such material as monosilane is thermally decomposed and deposited on a surface of the silicon substrate), a method in which an oxide film is formed by anodic oxidation, various kinds of vapor deposition such as sputter vapor deposition, and a method of oxidizing in plasma. However, these methods also have the same problems in terms of film quality and film-thickness controllability.
In particular, the increase of the leak current density not only causes various problems such as the increase of power consumption of the device, the increase of operating temperature, the deterioration of stability, and the like, but also destabilizes the operation of the device if the amount of leak current is substantially equal to the amount of drain current.
In view of these problems, the inventors of the present invention invented a method of forming an oxide film by chemical oxidization and filed a patent application (patent publication 1: Japanese Laid-Open Patent Publication, Tokukai 2002-64093 (publication date: Feb. 28, 2002)). According to this method, a chemical oxide film is formed by soaking a silicon substrate into, for example, concentrated nitric acid. Then, the chemical oxide film is heat-processed in an inactive gas such as nitrogen. The heat processing decreases the leak current in the oxide film. This method is POA (postoxidization annealing), because the heat processing is performed after the oxide film is formed.
The inventers of the present invention invented another method of forming an oxide film by chemical oxidization and filed a patent application (patent publication 2: Japanese Laid-Open Patent Publication, Tokukaihei 9-45679 (publication date: Feb. 14, 1997)). According to this method, a chemical oxide film is formed by soaking a silicon substrate into, for example, concentrated nitric acid. Then, a metal film (e.g. platinum) having a function of oxidation catalyst is formed on the oxide film. After that, the oxide film is grown by heat processing in an oxidizing atmosphere.
The inventers of the present invention invented yet another method of forming an oxide film by chemical oxidization and filed a patent application (patent publication 3: Japanese Laid-Open Patent Publication, Tokukai 2002-57154 (publication date: Feb. 22, 2002)). According to this method, a chemical oxide film is formed by soaking a silicon substrate into, for example, concentrated nitric acid. Then, a metal film (e.g. platinum) having a function of oxidation catalyst is formed on the oxide film, followed by heat processing in an oxidizing atmosphere. Thereafter, the metal film and a part of the oxide film are removed by etching so as to reduce the thickness of the oxide film, and an electrode is formed on the oxide film.
According to the method of patent publication 1, the heat processing in the inactive gas is required to be performed at a relatively high temperature. If the heat processing in the inactive gas is performed at a high temperature, there is a problem that the thickness of the oxide film increases due to a very small amount of an oxidizing species, such as vapor and oxygen, mixed in the inactive gas.
Another problem of the method of patent publication 1 is that diffusion of the dopant is caused by the heat processing at a high temperature as in the case of high-temperature thermal oxidization, resulting in the destruction of shallow junctions. Moreover, there is also a problem that the heat processing at a high temperature cannot decrease the leak current density with sufficient reproducibility.
The method of patent publication 2 is not suitable for forming an extremely thin oxide film, because the method has a step of growing the oxide film. The method of patent publication 2 cannot effectively decrease the leak current density of the oxide film, either.
According to the method of patent publication 3, it is difficult to control the thickness of the oxide film, because it is necessary to reduce the thickness by etching. Moreover, if some parts of the oxide film are extremely thin as a result of etching, the leak current density increases. Therefore, it is difficult to decrease the leak current density with sufficient reproducibility.
Incidentally, the gate oxide film of a TFT has been conventionally formed by CVD (chemical vapor deposition) in which deposition is caused at a substrate temperature of about 600° C.
In order to manufacture a flexible liquid crystal display, the TFT is required to be formed on a substrate of organic material such as PET (polyethylene terephthalate). For this purpose, the TFT must be formed at a low temperature of not higher than 200° C. However, in order to deposit the silicon dioxide film by CVD, it is necessary to heat the substrate to a high temperature of 400° C. to 500° C. Thus, the deposition of silicon dioxide film by CVD is not suitable for the formation of TFT in manufacturing a flexible liquid crystal display.
In the above-described TFT, in general, a relatively high voltage is applied to the gate electrode. Therefore, in order to prevent dielectric breakdown, the silicon dioxide film used as the gate oxide film needs to have a sufficient thickness.
The present invention was made in view of the foregoing problems. An object of the present invention is therefore to provide (i) a method of forming an oxide film on a surface of a semiconductor substrate, by which a high-quality (with low leak current density), extremely thin oxide film can be formed on a surface of a silicon substrate with excellent film-thickness controllability and at a low temperature, (ii) in particular, a method of forming a silicon dioxide film on a surface of a silicon substrate, by which a high-quality (with low leak current density), extremely thin silicon dioxide film can be formed on a surface of a silicon substrate with excellent film-thickness controllability and at a low temperature, and (iii) a method of manufacturing a semiconductor device by these methods. Another object of the present invention is to provide a method of forming a silicon dioxide film at a low temperature, so that the silicon dioxide film can be formed on a substrate of organic material such as PET.